1. Field of the Invention
This invention relates to the atmospheric flaring of one or more gas streams that are generated in an industrial plant. In particular, this invention relates to the flaring of one or more gas streams from a chemical plant.
2. Description of the Prior Art
Although this invention will, for sake of clarity and brevity, be described in relation to a polymer production plant wherein olefins are joined together to form polyolefins, this invention is not so limited, it being applicable to essentially any plant that generates at least one gas stream suitable for flaring (combustion).
Periodically in the operation of an industrial plant, particularly chemical plants, one or more gas streams are generated that have no further use, and must be disposed of in an acceptable manner. Often this waste plant gas is combustible (flammable), i.e., has a British thermal unit (Btu) content suitable for disposal by combustion in the ambient atmosphere by use of a conventional flare assembly known in the art. However, the combustibility of such a plant gas stream can be too low for efficient combustion in the atmosphere, and/or combustion in an environmentally acceptable manner. For example, the Btu content of the plant gas can be so low it would, upon burning, have a much lower destruction efficiency and thus allow higher amounts of hydrocarbons than is acceptable to be released into the atmosphere.
Thus, a predetermined minimum Btu content target value (predetermined minimum Btu value) is set in which the plant gas to be disposed of must contain before it can be passed to a flare for combustion. Sometimes the actual Btu content of the plant gas stream is sufficient to meet this predetermined minimum Btu value. In this situation, the plant gas can be burned without the need for additional combustion enhancing fuel, e.g., natural gas, to raise the Btu content of that plant gas at least to the predetermined minimum Btu value. Other times the actual Btu content of the plant gas is below this predetermined minimum Btu value and additional flammability enhancing fuel must be added to the plant gas stream in an amount sufficient to raise its Btu content at least to meet this predetermined minimum Btu value. It all depends on the chemical make-up of the plant gas stream. However, this chemical make-up can vary widely and randomly particularly when the gas stream is a composite of a plurality of gas streams formed in separate parts of the plant, and then combined into a single flare gas stream.
Sometimes the actual Btu content of the gas stream is essentially the same (steady state or stable) as to flow rate (quantity) and chemical composition, and if this was always the case it would be relatively easy to calculate how much additional fuel, if any, was required to get the Btu content of the gas stream up to its particular predetermined minimum Btu value.
Other times, and sometimes often, the actual Btu content of the final gas stream to be combusted varies, and in an unpredictable manner since the stream can be composed of a number of different waste streams of varying chemical compositions and quantities. This random variance of characteristics of the final gas stream to be flared renders the maintenance of its predetermined minimum Btu value problematic.
One solution for this problem is to employ, based on plant history, a large excess of additional combustion enhancing fluid fuel so that the plant gas stream to be flared always has a final Btu content well above any level needed in the past, thereby, theoretically always meeting its predetermined minimum Btu value. However, this approach is based only on past performance, and some plants can generate gas streams in the future that have a lower Btu content than ever experienced in the past. Also, this approach is wasteful of added fuel when the actual Btu content of the gas stream is, from time to time, at or above its predetermined minimum Btu value, so this approach can prove expensive.
The challenge then is always to meet the predetermined minimum Btu content target value while using only the minimum amount of added fuel, and to do so whether the actual Btu content and other characteristics of the plant gas stream is stable or randomly varying in composition, flow rate, and the like.
This invention meets that challenge.